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      函数参数详解
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        <p>在开发过程中，有很多时候，开发者没搞清楚函数参数，而随意传递参数，这可能会给函数增加很多不必须的参数，那么这次来细说一下这个函数使用过程的参数的使用。<br><a id="more"></a><br>先看下面这个例子：<br><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">var</span> b = <span class="number">3</span>;</div><div class="line"><span class="function"><span class="keyword">function</span>(<span class="params">a = <span class="number">42</span>, b = a + b + <span class="number">5</span></span>)</span>&#123;</div><div class="line">	<span class="comment">// ..</span></div><div class="line">&#125;</div></pre></td></tr></table></figure></p>
<blockquote>
<p>b = a + b + 5 在参数b(=右边的b，而不是函数外的那个)的TDZ中访问b，所以会出错。而访问a却没出错，因为此时刚好跨出了参数a的TDZ。<br>在ES6中，如果参数被省略或者值为undefined，则取该参数的默认值：</p>
</blockquote>
<figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">function</span> <span class="title">foo</span>(<span class="params"> a = <span class="number">42</span>, b = a + <span class="number">1</span></span>)</span>&#123;</div><div class="line">	<span class="built_in">console</span>.log(a, b);</div><div class="line">&#125;</div><div class="line"></div><div class="line">foo();					<span class="comment">// 42  43</span></div><div class="line">foo(<span class="literal">undefined</span>)		<span class="comment">// 42  43</span></div><div class="line">foo(<span class="number">5</span>)					<span class="comment">// 5  6</span></div><div class="line">foo(<span class="keyword">void</span> <span class="number">0</span>, <span class="number">7</span>)		<span class="comment">// 42  7</span></div><div class="line">foo(<span class="literal">null</span>)				<span class="comment">// null 1</span></div></pre></td></tr></table></figure>
<blockquote>
<p>表达式中 a + 1 中的null被强制类型转换为0.</p>
</blockquote>
<p>对ES6中的参数默认值而言，参数被省略或被赋值为undefined效果都一样，都是取该参数的默认值。然而某些情况下，它们之间还是有区别的：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">function</span> <span class="title">foo</span>(<span class="params">a = <span class="number">42</span>, b = a + <span class="number">1</span></span>)</span>&#123;</div><div class="line">	<span class="built_in">console</span>.log(</div><div class="line">		<span class="built_in">arguments</span>.length,</div><div class="line">		a, b,</div><div class="line">		<span class="built_in">arguments</span>[<span class="number">0</span>],</div><div class="line">		<span class="built_in">arguments</span>[<span class="number">1</span>]</div><div class="line">	)</div><div class="line">&#125;</div><div class="line">foo();						<span class="comment">// 0  42  43  undefined undefined</span></div><div class="line">foo(<span class="number">10</span>);					<span class="comment">// 1  10  11 10  undefined</span></div><div class="line">foo(<span class="number">10</span>, <span class="literal">undefined</span>);		<span class="comment">// 2  10  11 10  undefined</span></div><div class="line">foo(<span class="number">10</span>, <span class="literal">null</span>);			<span class="comment">// 2  10  null  10  null</span></div></pre></td></tr></table></figure>
<p>虽然参数a和b都有默认值，但是函数不带参数时，arguments数组为空。<br>相反，如果向函数传递undefined值，则arguments数组中会出现一个值为undefined的单元，而不是默认值。</p>
<p>ES6参数默认会导致arguments数组和相对应的命名参数之间出现偏差，ES5也会出现这种情况：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">function</span> <span class="title">foo</span>(<span class="params">a</span>)</span>&#123;</div><div class="line">	a = <span class="number">42</span>;</div><div class="line">	<span class="built_in">console</span>.log(<span class="built_in">arguments</span>[<span class="number">0</span>]);</div><div class="line">&#125;</div><div class="line">foo(<span class="number">2</span>);		<span class="comment">// 42 (linked)</span></div><div class="line">foo();			<span class="comment">//undefined (not linked)</span></div></pre></td></tr></table></figure>
<p>向函数中传递参数是，arguments数组中的对应单元会和命名参数建立关联（linkage）以得到相同的值。相反，不传递参数就不会建立关联。</p>
<p>但是，在严格模式中并没有建立关联这一说：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">function</span> <span class="title">foo</span>(<span class="params">a</span>)</span>&#123;</div><div class="line"><span class="meta">	"use strict"</span>;</div><div class="line">	a = <span class="number">42</span>;</div><div class="line">	<span class="built_in">console</span>.log(<span class="built_in">arguments</span>[<span class="number">0</span>]);</div><div class="line">&#125;</div><div class="line">foo(<span class="number">2</span>);			<span class="comment">// 2 (not linked)</span></div><div class="line">foo(); 			<span class="comment">// undefined (not linked)</span></div></pre></td></tr></table></figure>
<p>因此，在开发中不要依赖这种关联机制。实际上，它是JS语言引擎底层实现的一个抽象泄漏（leaky abstraction），并不是语言本身的特性。</p>
<p>arguments数组已经被废止（特别是在ES6引入剩余参数…之后），不过它并非一无是处。</p>
<p>在ES6之前，获得函数所有参数的唯一途径就是arguments数组。此外，即使将命名参数和arguments数组滥用也不会出错，只需遵守一个原则，即「不要同时访问命名参数和其对应的arguments数组单元」。</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">function</span> <span class="title">foo</span>(<span class="params">a</span>)</span>&#123;</div><div class="line">	<span class="built_in">console</span>.log( a + <span class="built_in">arguments</span>[<span class="number">1</span>]); <span class="comment">//安全</span></div><div class="line">&#125;</div><div class="line">foo(<span class="number">10</span>,<span class="number">32</span>); <span class="comment">// 42</span></div></pre></td></tr></table></figure>

      
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